a. Field of Invention
The invention relates to strain sensing technology and, more particularly, to a hybrid fiber optic sensing system (FOSS) for high spatial resolution and high frequency strain sensing measurements.
b. Background of the Invention
The prediction and monitoring of the strains acting on a vehicle or structure during its operation are important engineering tools that have the ability to greatly improve vehicle or structure safety and performance. Fiber optic sensors represent some of the most mature technology for obtaining active environment measurements such as strain and temperature. FOSS technology is particularly suited for aerospace applications in that the sensors are lightweight, accurate within a wide temperature range, and immune to electromagnetic and radio-frequency interference and radiation. FOSS-based sensors therefore have a relatively long lifespan, especially as compared to conventional foil strain gage sensors, and may be embedded into the composite structure of an aircraft or other vehicle or into a stationary structure during manufacturing to optimally measure active strain data during the lifespan of the system. Also, due to these sensors' wide range of elasticity under environmental perturbation, they may be used for failure testing during the design phase of a vehicle or structure to ascertain the limits of the system in operation.
One conventional type of fiber optics-based sensor is known as the Fiber Bragg Grating (FBG or “Bragg grating”). When incorporated into an optical fiber, an FBG reflects particular wavelengths of light based on its Bragg wavelength, an inherent characteristic of the FBG for a given mode. Strain or certain other forces acting on the fiber and thus on the FBG will alter the reflected wavelength. The characteristics of the reflected light can thus be analyzed to determine the strain characteristics of the fiber, with data points corresponding to the discrete locations of the Bragg gratings.
A representative FBG is depicted in FIG. 1. Fiber 1 has a core 2, typically made of silica and having a refractive index n2. The FBG is represented by area 3 comprising a plurality of individual index variations 4, shown in shading along core 2. Each index variation 4 has been modified by UV light or the like to alter its refractive index to n1. As described below, a single fiber may have multiple FBGs along its core 2. Light traveling along core 2 and passing through FBG 3 is partially reflected at each index variation 4. Where light has the characteristic Bragg wavelength of a given FBG, reflections at each index variation 4 are propagated back down the fiber. The grating period refers to the layout and spacing of the index variations 4.
Fiber optic sensing technology to measure parameter data conventionally falls into one of two categories: Optical Frequency Domain Reflection (OFDR) and Wavelength-Division Multiplexing (WDM). In a WDM sensor array system, a source emits a light covering a selected wavelength range. The light is coupled into the fiber containing an array of serially multiplexed sensor gratings, each having a different Bragg wavelength. The WDM system is designed so that the Bragg wavelengths of each sensor are separated from one another by a certain wavelength range so that the FBGs don't interfere with each other. However, the wavelength range of reflections from a given FBG combined with the finite bandwidth of the light source means that only a limited number of FBGs can be used in a single fiber, thus limiting the spatial resolution of strain measurements in a WDM system. WDM systems typically have relatively high sample rates in terms of kHz ranges.
By contrast, OFDR allows for a much higher number of gratings per fiber (thousands of gratings per fiber are permitted) but is characterized by a much lower sample rate (i.e., less than 100 Hz) than WDM. Thus, with OFDR, higher spatial resolution is available but is acquired much less frequently. In an OFDR system, all of the FBGs in a single fiber have the same central wavelength, and their positions and characteristics along the fiber are detected by measuring the beat frequency of an individual grating's reflection against the reflection from a reference arm of an interferometer having a known length. Because only a narrow band of light is supplied, an optical fiber in an OFDR system can effectively be “continuously grated” with FBGs having similar characteristics. However, current technology limitations prevent OFDR systems from having a high sample rate and therefore applications for OFDR systems are limited to lower frequency applications.
FOSS technology based on OFDR has a wide range of applications. Applications include the use of FOSS sensors on commercial and non-commercial aeronautics and AeroStructures, on rotary blades for helicopters and other rotorcraft, on high performance automobiles and/or in the racing industry, and in the fields of civil and mechanical engineering to measure strains on bridges, buildings, wind turbines, and the like. In aerospace applications, strain sensing technology must be lightweight and have a small footprint, both in terms of the sensor array and the analytics technology, it must be able to be securely attached to or embedded in the structure of the aircraft, and it must function across a wide temperature range.
The ability of engineers to capture structural strain measurements during high frequency shock and/or vibration events would enable better modeling of the structural capacities of a structure during extreme conditions, to predict the fatigue of the structures during normal operating conditions, and to gain other important information about the system in operation or at failure. High frequency sampling (1 kHz or more) is necessary to capture such measurements. However, it is also necessary to maintain the spatial resolution available under low-frequency methods like OFDR.
Therefore, it would be advantageous to provide an optical strain sensing system for the active capture of strain sensing data which provided both a high spatial resolution and a high frequency rate of measurement. It would also be desirable to provide such a system in a single, integrated unit that is capable of receiving and processing both high frequency and high spatial resolution strain data in real time. In addition, it would be advantageous for this system to be able to interface with conventional strain sensing analytics technologies.